Control system



Feb. 19, 1946. J.G.TRAW1CK 2,395,351

CONTROL SYSTEM Filed June 12, 1945 VIII/111111111 Y n v INVENTOR. JACK G. TRAWICK Patented Feb. 19, 1946 UNITED STATES FATENT OFFICE CONTROL SYSTEM Jack G. Trawick, Birmingham, Ala., assignor to The Brown Instrument Company, Philadelphia,

Pa a corporation of Pennsylvania Application June 12, 1943, Serial No. 490,629

12 Claims.

The present invention relates to pulp digesters, and more particularly to a system for controlling the relief of gases from the digester during the digesting process.

In the process of operating a digester, the digester is first filled with a charge of wood chips and liquor. Thereafter it is sealed and steam is added to bring the temperature of the charge up to a predetermined degree at a given rate. While the temperature is being raised gas is given on as a result of chemical action of the process, and the digester has to be vented from time to time to prevent too great an increase in pressure therein.

The Venting or gassing off causes violent bubbling of the charge in the digester. This is desirable, but at the same time may carry chips and pulp against a screen in the vent pipe, which screen acts to keep the escaping gases clean. When the screen becomes clogged it is customary tosuddenly introduce an amount of steam through the vent opening to clean off the screen so that the gassing on may continue ina normal manner.

In the operation of this invention there is provided a means that is automatically responsive to the clogging of ascreen in the digester relief pipe which means serves to intermittently open a steam valve. Steam from a high pressure source i then forced through the screen to clean it. It is therefore an object of the invention to con trol the gassing off of a paper pulp digester. A further object is to automatically open a valve in a steam supply pipe connected to the vent pipe of a digester to supply steam for the purpose of clearing the vent pipe. It is a further object of the invention to control the gassing off process of a digester by providing means which is automatically responsive to the clogging up of the screen in the digester relief pipe to clean the screen and open this pipe. It is a, further object of the invention to repeat the unclogging operation periodically until such time as the relief system of the digester is again functioning normany.

The various features of novelty which characterize this invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages and the specific objects obtained with its use, reference should be had to the accompanying drawing and descriptive matter in which is illustrated and described a preferred embodiment of the invention.

In the drawing:

Figure 1 is a diagrammatic showing of the digester control system of this invention, and

Figure 2 is awiring diagram showing the manner of operating the solenoid valves of Figure 1.

Referring to Figure la there is shown a digester I that is provided with a removable top 2 through which the digester may be charged, and with a pipe 3 through which the pulp may be discharged when the digesting process is completed. This latter pipe is provided with a valve 4 that is operated to control the discharge. Steam for the digesting process is supplied to the lower part of the digester through a perforated pipe 5 that has in it an air operated valve 6 to control the flow through the same.

The valve 6 is operated in accordance with a predetermined time temperature schedule so that the cooking of the chip may be carried out in a given manner. To thi end there is provided a pneumatic transmitting instrument 1 which is responsive to the temperature of the digester as measured by a'thermometer bulb B that is located at a suitable point therein. This instrument is supplied with air under a constant pressure through a pipe 9 and serves to set up an air pressure in a pipe ID that is proportional to the temperature of the digester. The instrument 1 may well take the form of a pneumatic transmitting instrument disclosed in Moore Patent 2,311,853, granted on February 23; 1943. The air in pipe II) that is proportional to the temperature of the digester is applied to a time temperature controller II which serves to set up an air pressure in a pipe l2 that is proportional to the digester temperature and varies the pressure in accordance with a definite time schedule. Such instruments are common in the art. The pressure in pipe [2 is applied to the diaphragm of the control valve 6. i

\ From the above it will be seen that the cook ing steam supply to the digester will be varied in such a manner that a definite cooking schedule will be maintained so that the digesting process will be carried out in a predetermined manner. As the digesting process is continued gases will be given off from the charge. In order to prevent the pressure in the digester from rising to a dangerous point, and to better control the digesting process, there is provided a vent or relief pipe 13 in which is located an air operated control valve I4. This valve is of the normally closed type which is opened varying amounts by the application of an air pressure to its di aphragm. During the time that the disaster is operating a violent boiling often takes place as the gas is produced. This will cause chips to be carried out of the digester along with the vented gas unless some means is used to prevent this from occurring. Accordingly a screen I5 is placed over the entrance of the vent pipe I3 in the neck of the digester.

It is usually the practice to maintain a pressure in the top of the digester which is equal to that of saturated steam at the temperature of the digester. To this end the relief valve is controlled in response to both the temperature and the pressure therein. A pneumatic transmitting in- I strument I6, similar in construction to instrument "I, is responsive to the pressure in the digester as measured through a pipe II that terminates in the upper part thereof in the gas space above the liquid level. This instrument is upplied with air under constant pressure through a, pipe I8 and applies a variable air pressure, depending pressure from the transmitter "I through a pipe U 23, which is a branch of pipe I8, to the control point adjusting provisions of instrument 2|. Thi latter instrument may well take the form of that disclosed in Moore Patent 2,216,448 granted on October 1, 1940. Both control instruments II and 2| are supplied with air under a suitable pressure through pipes 24 and 25 respectively. In practice pipes 9, I8, 24, and 25 would all be connected to the same air supply.

During normal operation of the digester steam is supplied through the pipe 5 in accordance with a predetermined time temperature schedule to cook the chips. As the cooking proceeds and gas is accumulated in the upper part of the digester the valve I4 is operated to vent the gas and keep the pressure at the proper value for the temperature at the time. From time to time, however, the strainer I5 may become clogged with chips and pulp to prevent escape of gas through pipe I3 and relief of the pressure, even though valve 1 I4 may be open. A means is therefore provided to clean the strainer I5 and mechanism which is. responsive to clogging of the strainer is provided to operate this means.

Connected to the vent pipe I3 and in a direct linewith the strainer I5 is a steam pipe 2'! which has an air operated valve 28 in it to control the flow of steam. This valve i of the normally closed type and is opened by applying air pressure to its diaphragm to permit steam to blow through the pipe and against the strainer to clean the same. Air is supplied to the valve 28 through a pipe 29 that has a solenoid valve 3| in it.

To detect clogging of the screen and initiate a response thereto a temperature controller 32, shown diagrammatically in Figure 2, is used. This controller has a temperature responsive bulb 33 that .is located in the vent pipe I 3 just beyond the valve I4. Normally during the gassing off process, the expansion of the gas across the valve I4 from digester pressure to a pressure substantially atmospheric cools the gas and the thermometer bulb immersed therein. When the strainer I5 becomes clogged the gas will stop flowing through pipe I3 and its cooling effect on the bulb 33 will not occur. The temperature of the bulb will therefore rise, due partially to conduction of heat from the digester. This raising of the temperature of bulb 33 is used to close valve I4 and open valve 28. The manner in which this is accomplished is disclosed in Figure 2.

The bulb 33 forms part of 'a closed thermometric system which includes a bellows 34 that is located in the controller 32. As the bulb increases in temperature the bellows 34 will be expanded to move a supporting member 35 around it pivot 38 against the force of a spring 31. This support carriesa mercury switch 38 that is normally open, but which will be'closed when the bellows expands. Switch 38 is in the circuit of a single revolution motor 39 so that when the switch is closed the motor field 4| will be energized. Immediately after the motor starts, a cam 42 on its shaft will permit a switch 43 to close a circuit parallel to that of switch 38 so that the motor will at least make one revolution until cam 42 again opens switch 43. At this time the motor will stop unless switch 38 is still closed. Also carried by the motor shaft is a contact arm 44 that engages a segment 45 which extends for approximately 93". When the arm 44 engages segment 45, which it does immediately after the motor starts, parallel circuits including a solenoid 46 of valve 3| and a solenoid 41 of a valve 48 in pipe 22 will be closed to energize these solenoids and operate the valves.

Each of the solenoid valves 3| and 48 is a three way valve that in one position will connect the diaphragms of air operated valves 28 or I4 to' it vents the diaphragm of valve 28 to the atmos-' phere, but when energized will close the vent to permit air to flow through pipe 29 to open Valve 28. The valve 48 is in pipe 22 and is normally held in the position shown by a spring 49 so that 7 air from the instrument 2| can operate valve I4. When the solenoid 41 is energized the valve 48 is adjusted to vent valve I4 to the atmosphere and permit the latter valve to close. V

In the operation of the system, the gas escaping from the digester will cool the bulb 33 s that bellows 34 will contract and open switch 38. With this state of affairs the'solenoid valves will be.

in the positions shown so that valve 28 is closed, and pipe 22 is open with the relief valve I4 being adjusted by the controller 2|. When the screen I5 becomes clogged bulb 33 will heat up and bellows 34 will expand to close switch 38 to start motor 39 running. During the first part of the revolution of motor 39, contact 44 will engage segment 45 to energize valves 3| and 48. This will permit valve I4 to close and will cause an opening of valve 28 so that steam may flow through pipe 21 to fiush'the strainer I5. 'The flushing operation takes place during approximately only one quarter of the cycle of rotation of motor39 and the solenoid valves return to the position shown after the arm 44 runs off segment 45. If the steam blown against the strainer was sufiicient to clean it, gas willbe escaping from the digester during the remainder of the rotation of the motor. This escaping gas Will be sufficient to cool down the bulb 33 so that switch 38 will open, and when switch 43 is again opened by cam 42 the motor will stop. If, however, the steam blown against the strainer was not sufficient to clean it, the switch 38 will not be open at the end of the cycle so that motor 39 wil1 continue to rotate and aeeassr again produce an Opening ofval've 28 so thatmore steam will be blown against the screen,

Thus it will be seen that upon clogging of the screen it steam will automatically be blown against the same to clean it. If the first amount of steam is not sufficient more steam will be blown against the screen a predetermined time later, and this will periodically be repeated until the screen It is cleaned.

A switch is located in the wire between the line and the motor, which switch may be opened to prevent operation of this portion of the system when desired, Generally the switch 51 will be kept open when the digesting process is started and until such time as gas is produced and the. Valve l 5 begins to operate. In some cases, however, it may be desirable to keep the switch 5i closed so that the strainer cleaning system can be used to supply supplementary steam to the digesterat the start of the process.

From the above description it will be seen that I have provided mechanism that is responsive to failure of the relief of gases from a paper pulp digester to correct this situation and start thedigester relief system operating again in a normal fashion. It will be apparent to those skilled in the art that this system may be used in other places in which it is desired to maintain a constant flow of gas through a passage that may at times become obstructed.

While in accordance with the provisions of the statutes, I have illustrated and described the best form of my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of my invention as set forth in the appended claims, and that in some cases certain features of my invention may sometimes be used to advantage With0ut "a corresponding use of' other features.

Having now described my invention, what I claim as new and desireto secure by Letters Patent is:

1. In a digester having a relief passage provided with a strainer through which gas may pass and a pipe through which steamrnay pass to clean the strainer when it becomes clogged, a first valve in said relief passage to control the passage of relief gases, means located in said relief passage beyond said first valve and responsive to the temperature of the relief gases escaping across said first valve, a second valve in said pipe to control the flow of steam passing to said strainer, and control means operated by said responsive means when the latter is subjected to a predetermined temperature that is indicative of a reduced fiow of gas to simultaneously cause said first valve to close to shut off said relief passage and said second valve to open to let steam flow against said strainer.

2. In a digester having a relief passage provided with a strainer through which gas may pass and a pipe through which steam may pass to clean the strainer, a first valve in said relief passage, temperature responsive means located in said relief passage and responsive to the temperature of the gas therein, a second valve in said pipe to control the passage of steam, means to operate simultaneously said valves to close said first valve and open said second valve, and mechanism operated by said temperature responsive means to control the operation of said means to operate said valves upon the attainment of a predetermined temperature in said relief. pas sage.

3. In a digester having a relief passage provided with a strainer through which gases from the digester may pass and a pipethrough which steam may be passed to I clean said strainer, a first valve in said relief passage, 2, second valve insaid steam pipe, electrical means to control.

the operation of said'valves, cyclically operating means operative to actuate said electrical means for a portion of each cycle thereof to close said first valve and to open said second valve whereby said relief passage will be closed and said steam pipe will open to force steam against said strainer toclean the same, and temperature responsive control means having a portion located in said relief passage operative upon a predetermined temperature in said passage to initiate operation of said cyclicallyhoperating means,

4. In a digester having a relief passage pro vided with a strainer through which gasmay pass and a pipe through which steam to clean the strainer passes, a first valve in said relief passage, a second valve in said pipe, means to con trol the operation of said valves, motor driven means operable to actuate said means to control for a portion of a given period of time to close said first valve and to open said second valve,

and temperature responsive control means having a portionthereof located in said relief pas sage to actuate said motor driven meansupon the occurrence of a predetermined temperature in said relief passage.

5. In a digester having a relief'passage pro vided with a strainer through which gas may pass and a pipe through which steam to clean the strainer may pass, a first valvein said relief passage, a sec-0nd valve in said pipe, means to control the actuation of said valvesto close said first valve and open said second valve, motoroperated means to actuate said means to control for a portion of a given period of time, temperature responsive control means having a part thereof located in said relief passage and operative upon the occurrence of apredetermined temperature in said relief passage to start said motor operated means, and means formin part of said motor operated means tostop the operation of the same if the temperature in said relief pas .sage differs from said predetermined temperature after a period of operation.

6. In a digester having a relief passage provided with a screen through which gas may pass, the combination of a valve in said relief passage, means to operate said valve to control the passage of gas therethrough, control means having a part thereof located in said passage beyond said valve and responsive to the temperature of gas passing through the same and operative upon the attainment of a predetermined high temperature therein, a supply of steam to clean said screen when'it becomes clogged, a valve to control said supply of steam, and means operated by said control means to open the valve controlling said supply of steam upon the occurrence of said predetermined high temperature.

7. In a digester having a relief passage and a strainer protecting the entrance of the same, a valve in said relief passage, means to control the operation of said valve, control means having a part thereof located in said passage and operative in response to a predetermined reduction of the flow of gas therein, a supply of steam to open the strainer when it becomes clogged, a valve to control the steam supply, and means operatively connected to both of said valves and operated by said control means to close said first mentioned valve and to open said second mentioned valve upon the occurrence of the predetermined reduction of the flow of gas. I

,8. In a digester having a relief passage and a strainer protecting the entrance of the same, a valve in said passage to control the flow therethrough, a supply of fluid under pressure to be forced against said strainer to open the same when it becomes clogged, a second valve to control said supply of fluid, control means having a part located in relief passage and operative in response to a reduction of the fiow of material therethrough, means to control the closing of said first valve and means to control the closing of said second valve, and mechanism operated by the control means to operate said means controlling said valves, the arrangement being such that upon stoppage of flow in said relief passage the valve .therein will be closed and the second valve will be opened, said mechanism including means to render the operation of said valves periodic.

9. In a digester having a relief passage and a screen at the entrance of said passage to clean material passing therethrough, the combination of a first valve in said relief passage, means responsive to a physical condition of material in the digester to operate said valve, a suppl line for cleaning fluid for said screen, a second valve in said supply line, control means having a part located in said relief passage responsive to passage of material through the same, cyclically operated mechanism under control of said control means and operatively connected to said first and second valves operative to remove the control of said first valve from said means responsive to a physical condition of the material and close the same and to open said second valve for a portion of each cycle thereof, said mechanism including means to repeat the cycle if said screen is not opened by the passage of cleaning fluid past said second valve as shown by the passage of fluid past said part when said first valve is again open.

10. In a di'gester having a relief passage and a strainer over the entrance of the same to clean material passing therethrough and a pipe for steam to clean said strainer when it becomes clogged, a first valve in said relief passage, a

second valve in said pipe, control means having a part responsive to flow of material through said relief passage and operated in accordance with the flow therein, and mechanism operative by said control means in response to stoppage of material flowing through the relief passage, and connecting means between said mechanism and valves to close'said first valve and open said second valve, whereby steam may flow through said pipe to clean said screen,

7 11. In a digester having a relief passage and a strainer over the entrance of the same to clean material passing therethrough and, a pipe for steam to clean said strainer when it becomes clogged, a first valve in said relief passage, a second valve in said pipe, control means having a part responsive to flow of material through said relief passage and operated in accordance with the flow therein, a first solenoid valve operative when energized to cause closure of said first valve, a second solenoid valve operative when energized to cause opening of said second valve, means to energize said solenoid valves, and cyclically operated mechanism operatively connected to and controlled by said control means to operate said means to energize said solenoid valves for a portion of each cycle.

12. In a digester having a relief passage with a strainer over the entrance thereof to keep material passing throughthe same clean and apipe to supply cleaning fluid for said strainer, the combination of a first 'air operated valve in said relief passage, a second air operated valve in said pipe, separate air lines to supply air to said valves to open the same, a first solenoid valve operative when energized to cut off air to the first air operated valve so said first air operated valve will close, a second solenoid valve in the air line for said second air operated valve operative when energized to open said air line so that air may open said second air operated valve, control means having a part in said relief passage responsive to flow of material therein, means to energize said solenoid valves, and mechanism operated by said control means to operate said means to energize said solenoid valves when no material is passing through said relief passage, said mechanism being cyclic in operation.

JACK G. TRAW'ICK. 

